This invention relates to servo track following of servo edges of dissimilar servo signals recorded on magnetic tape, and, more particularly, to calibrating indexed servo positions displaced with respect to the servo edges.
Magnetic tape data storage typically provides prerecorded servo tracks to allow precise positioning of a tape head which has servo sensors, with respect to the prerecorded servo tracks. The tape head comprises one or more read/write elements precisely positioned with respect to the servo sensors and which trace data tracks parallel to the servo tracks. One example of a magnetic tape system is the IBM 3590, which employs magnetic tape having prerecorded servo patterns that include three parallel sets of servo edges, each servo edge being an interface between two dissimilar recorded servo signals, each set of servo edges comprising one servo edge on each of opposite lateral sides of a middle recorded servo signal.
The tape head has several spaced apart servo sensors for each servo edge, with the result that the tape head may be stepped between the servo sensors, each positioning the read/write elements at different interleaved groups of data tracks.
Typically, for a given servo pattern of a set of two servo edges, the outer servo signals are recorded first, and the center servo signal is recorded last, to provide the servo edges. As pointed out by the incorporated ""159 patent, the nominal separation distance between the servo edges of each set of servo edges is a certain distance, such as 80 microns, but there is variation in the magnetic separation between the servo edges, for example, due to the variation of the width of the physical write element which prerecords the servo pattern, due to variation in the magnetic characteristics of the physical write element, etc. The variation may occur between servo tracks in a single magnetic tape, and may occur between prerecording devices and therefore between magnetic tapes.
To reduce the apparent difference of the edge separation distance of the prerecorded servo tracks from nominal, the prerecording of the servo tracks is conducted at different amplitudes so as to attempt to compensate for the physical difference and provide a magnetic pattern that is closer to nominal. Thus, the difference in physical distance and the amplitude compensation may tend to offset each other with respect to the apparent distance between the servo tracks. These actions may provide an adequate signal for track following at the servo edges.
However, to increase track density, a servo sensor may be indexed to positions laterally offset from the linear servo edges to provide further interleaved groups of data tracks. The indexed positions are determined by measuring the ratio between the amplitudes of the two dissimilar recorded servo signals. Thus, when the amplitudes of the recorded servo signals are varied to compensate for physical distance variations, track following the prerecorded servo edges at the offset indexed positions becomes less precise. As the result, the data tracks may vary from the desired positions, for example, squeezed together, such that writing on one track with a write element that is subject to track misregistration (TMR) may cause a data error on the immediately adjacent data track. Commonly assigned U.S. patent application Ser. No. 09/703,905, filed Nov. 2, 2000, illustrated the use of curve fitting for servo calibration to enhance precision.
The tape path of the above IBM 3590 is a guided tape path, limiting the lateral movement of the magnetic tape so that the guiding noise is small enough that the step from one ratio to another was discernible. Another approach is required for open channel guiding in which the magnetic tape can move laterally a distance which is substantially greater than that between index positions, thereby introducing substantial noise into the guiding process. The guiding signal to noise ratio thus becomes negative, with the guiding noise being far larger than the step from one ratio to another, making it impossible to gather data points with a monotonic slope to conduct a calibration of the servo ratios.
An object of the present invention is to calibrate the servo index positions which are laterally offset from servo edges recorded on magnetic tape in an environment where the magnetic tape is subject to movement in the lateral direction.
A tape drive servo system and method are provided for calibrating servo index positions of a magnetic tape. The magnetic tape has at least one set of parallel linear servo edges, wherein each servo edge comprises an interface between two dissimilar recorded servo signals, and each set of servo edges comprises one of the servo edges on each of opposite lateral sides of a middle recorded servo signal. The servo calibration is of the servo index positions which are laterally offset from the linear servo edges and are measured by the ratios of the dissimilar recorded servo signals.
The servo system comprises at least one servo sensor of a tape head, wherein the tape head is movable laterally of the magnetic tape, and wherein the servo sensor senses the recorded servo signals of the magnetic tape comprising at least one servo edge of dissimilar recorded servo signals. The servo system also comprises a servo detector coupled to the servo sensor for determining a ratio of the servo signals sensed by the servo sensor and providing digital servo signals at a predetermined sample rate; an independent position sensor to sense lateral position of the magnetic tape with respect to the tape head servo sensor; a servo loop for positioning the tape head laterally with respect to the magnetic tape to track follow the sensed servo signals at specific position error signals representing displacements from the linear servo edges as determined from ratios of the sensed servo signals as determined by the servo detector; and logic coupled to the servo detector, the independent position sensor, and the servo loop.
The logic operates the servo loop to inject a defined signal to modulate the lateral position of the head and, thereby, the servo sensor.
In one embodiment, the logic operates the servo loop to laterally position the servo sensor to sense the servo signals at continually altered digital set points of the ratios of the sensed servo signals. The set points are altered at the sample rate of the servo loop, and are altered to inject a predetermined sinusoidal pattern single frequency positioning signal, whereby the servo loop track follows the linear servo edges with the servo loop at the continually altered digital set points.
The logic digitally determines, from the independent position sensor at the sample rate, the lateral positions of the tape head servo sensor with respect to the servo edge locations of the magnetic tape; and digitally determines, from the servo detector of the servo loop, the ratios of the servo signals sensed by the servo sensor, and provides digital servo signals; converts the digitally determined independent position sensor lateral positions to frequency components; and converts the digitally determined ratios of the servo signals to frequency components; selects from the frequency components of the independent position sensor lateral positions, and from the frequency components of the ratios of the servo signals, the predetermined sinusoidal pattern single frequency and at least one harmonic thereof; and converts the selected frequency components to independent position sensor lateral positions, and converts the selected frequency components to ratios of the servo signals.
The logic fits a curve to the converted independent position sensor lateral positions of the selected frequency components with respect to the converted ratios of the servo signals to calibrate expected position error signals for the servo loop at the laterally offset servo index positions with respect to the sensed servo edge(s).
In one embodiment, the injected predetermined sinusoidal pattern single frequency positioning signal comprises a single frequency selected so that the single frequency and major harmonics thereof each differs from intrinsic operational frequencies of the track following servo system and/or tape system.
In one embodiment, the logic converts the digitally determined independent position sensor lateral positions to frequency components by conducting fast Fourier transforms (FFT) of the digitally determined independent position sensor lateral positions; and converts the digitally determined ratios of the servo signals to frequency components by conducting fast Fourier transforms (FFT) of the digitally determined ratios of the servo signals. The logic further converts the selected frequency components to independent position sensor lateral positions and the selected frequency components to ratios of servo signals, both by conducting inverse fast Fourier transforms (IFFT) of the selected frequency components.
For a fuller understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.